1. Field of the Invention
The field of art to which this invention pertains is a transistor amplifier and more particularly a transistor amplifier having a protective circuit.
2. Description of the Prior Art
Transistor amplifiers particularly power transistor amplifiers for amplifying audio signals have been provided with protective circuits for protecting a power transistor from being damaged when a load, for example, a loudspeaker is short-circuited. There have been proposed various types of protective circuits. For example, there are methods of sensing the heat of a power transistor, and decoupling its input stage or its load in response to excessive heating. This heat detection is reliable but slow to respond to overload. Other methods of detecting current flowing through the power transistor have been used, however if the load is capacitive, such methods are not effective. A method of eliminating the above defect is disclosed in U.S. Pat. No. 3,904,979, in which there is described an embodiment to protect a power transistor even when the load is capacitive.
Recently, field effect transistors (hereinafter referred to as an FET) having characteristics similar to that of a triode have been used as power transistors. In such an arrangement, as the voltage applied to its gate electrode is increased from a negative voltage to ground potential, the drain current is accordingly increased. In a transistor amplifier using an FET, the input signal is shunted by a control signal from an overload detection circuit, but it is undesirable to hold the gate electrode of the FET in the ground potential. The reason is that if the gate voltage is equal to ground potential, the drain current may increase to the point where the output transistor is destroyed.